"The world may be about to watch one of its last Olympic Games without genetically enhanced athletes," wrote H. Lee Sweeney, a professor at the University of Pennsylvania School of Medicine.

Time to cherish the old-fashioned athletes of Sochi? It might be too late. Sweeney actually penned that warning a decade ago, in a Scientific American article exploring a little-known practice called gene doping.

In 2004, gene doping was not yet a real risk. But now? The technology has advanced, and authorities have reason to be concerned.

How gene doping works

If you've heard of gene therapy — which aims to cure diseases caused by genetic mutations by injecting cells with healthy DNA — gene doping is the same thing, but with a different end goal.

Theoretically, athletes could inject specific genes — say, those that promote muscle growth — to enhance their physique and performance.

Scientists now understand how genes work well enough that they have created "super mice," whose muscular build, endurance, and speed all far exceed that of normal mice. In one 2008 study, genetically altered mice had "a remarkable degree of hyperactivity" and "could run for up to 5 km, at a speed of 20 m/min without stopping."

That's 25 times farther than normal mice could run at that speed.

The researchers go on to note that the "super mice" were also frighteningly aggressive, and that — perhaps fortunately — it's neither "ethical nor possible" at this point to introduce the same genetic changes in humans. But there are many different kinds of gene doping, and in 2014, some may well be possible in humans.

We can already "rewire" certain genes

"Although one might argue that there is a long way between applying gene doping/therapy to mice and applying it to humans, there are more reasons to make the idea of genetically modified super athletes not so far-fetched," cautioned researchers in a 2009 review in Clinical Biochemistry.

Athletes, apparently including Tour de France riders in the 1990s, have been using a protein called EPO to increase their supply of oxygen-carrying red blood cells, which support the muscles and boost endurance. Since EPO is also made by the body, it was very hard to detect whether athletes were supplementing their natural supply. An effective test has since been developed and authorities can now detect the synthetic form of EPO.

"But ... introducing the EPO gene would result in the body producing its own EPO," writes Tim Franks of BBC News. "Could this be an undetectable way of improving oxygen delivery?" These experimental genetic interventions may be more difficult — even impossible — to detect, since the EPO would be made by the body instead of in a lab. It would be indistinguishable from the EPO healthy people already have.

And anyone, the BBC reports, can now get the EPO gene on the Internet. In gene therapy, this EPO gene would likely be delivered into the genome using a virus that injects the gene into cellular DNA. But geneticist Philippe Moullier told the BBC that athletes may opt for a quick and dirty shortcut: "injecting the purified gene directly into your muscle."

The authors of the review paper in Clinical Biochemistry pinpointed how specific research into genetic targets could unlock a whole list of athletic superpowers — promoting everything from endurance and pain tolerance to speed and strength.

Possible — but dangerous

Messing with our genes is no small matter, and both gene therapy to treat diseases and gene doping to enhance performance are a long way off from being safe enough for widespread use in humans.

For example, in 1999, Jesse Gelsinger, an 18-year-old with a liver disease, died days after receiving an experimental gene therapy. The therapy had been deemed safe in mice, monkeys, and even one human, with flu-like symptoms expected as the primary side effect. But in Gelsinger — who some later argued should have been excluded from the trial — the therapy caused an immune overreaction and, eventually, complete organ failure.

In another early trial, children receiving an experimental genetic therapy for a severe immune system disease — so-called "bubble boy disorder" — later developed leukemia.

Clinical trials are also underway for genetic diseases that cause muscles to waste away — of particular interest to athletes wanting to bulk up. But while such research is advancing quickly, it's not yet market ready.

Even with these small gains, the dangers associated with gene doping are very real. In the late 1990s, when scientists tried giving extra EPO genes to monkeys and baboons, their blood became "so thick," Sweeney wrote, "that it had to be regularly diluted to keep their hearts from failing."

So, are there super-athletes at Sochi?

WDR, a German broadcaster whom The Guardian noted "has a strong track record in investigating doping in sport," has reported that something that seems like a gene doping substance is floating around Sochi.

A scientist reportedly told the WDR correspondent that the substance, called "full-size MGF," "works two times faster than a normal muscle tonic and cannot be detected by the doping authorities."

While we were not able to independently verify WDR's report, Mario Thevis, a forensic chemist at the Center for Preventive Doping Research at the German Sport University Cologne, tested the substance. He confirmed to Science magazine that "full size MGF" contained a "variant of the IGF-1 gene," which "can prompt muscle growth." Using current anti-doping screens, Thevis said, it would be "more or less invisible."

"We were dealing with a highly pure and therefore probably highly dangerous substance," he added.

But are any athletes actually using it?

"There is no conclusive evidence that gene doping has been practiced in sport," noted researchers in the British Journal of Sports Medicine in 2013, but "given that gene therapy techniques improve continuously, the likelihood of abuse will increase."

The World Anti-Doping Agency, responsible for implementing the international standards and tests that aim to keep sports free of artificial enhancements, has officially banned gene doping since 2003. Now they are "devoting significant resources and attention to ways that will enable the detection of gene doping."

The technology that would make gene doping possible is getting there, and scientists are trying to stay one step ahead of athletes. No one can say yet whether they're succeeding.

After all, the Olympics are a once-every-four-years shot. Athletes may have the pressure of an entire country on their shoulders. The technology for gene doping is not widely accessible or reliably effective, and the risks may be great — but that doesn't mean athletes aren't clued in to the possibilities.

Sweeney told the BBC that after conducting mouse studies that used injected genes to promote muscle growth, he was immediately contacted by athletes and coaches — even though the research was in its earliest stages, targeted mainly toward people who were already very ill.

"I hadn't even considered the fact that a young, ultra-healthy individual who's competing at the peak of their career would risk anything," said Sweeney. "But obviously many of them would risk everything."